Alopecia areata is a disease that affects the hair follicles, which are part of the skin from which hairs grow. In most cases, hair falls out in small, round patches about the size of a quarter. Many people with the disease get only a few bare patches. Some people may lose more hair. The condition can affect anyone regardless of age and gender, though most cases occur before the age of 30. It is an autoimmune disease in which hair is lost from some or all areas of the body, usually from the scalp due to the body's failure to recognize its own body cells and subsequent destruction of its own tissue as if it were an invader. There is a slightly increased risk of having the disease if you have a close family member with the disease. It affects 2 percent of Americans (roughly 6.5 million people).
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https://www.jci.org/articles/view/44478. Journal of Clinical Investigation, Bald scalp in men with androgenetic alopecia retains hair follicle stem cells but lacks CD200-rich and CD34-positive hair follicle progenitor cells